Exploring, drilling and completing hydrocarbon wells are generally complicated, time consuming and ultimately very expensive endeavors. As a result, over the years increased attention has been paid to monitoring and maintaining the health of such wells. Premiums are placed on maximizing the total hydrocarbon recovery, recovery rate, and extending the overall life of the well as much as possible. Thus, logging applications for monitoring of well conditions play a significant role in the life of the well. Similarly, importance is placed on well intervention applications, such as clean-out techniques which may be utilized to modify downhole architecture and/or remove debris from the well so as to ensure unobstructed hydrocarbon recovery.
Following initial completions, the need to mill or drill-out downhole obstructions through interventional applications may arise. For example, it is not uncommon for regions of the well to naturally experience the buildup of scale and other debris which has a tendency to obstruct recovery and/or impede other downhole functionality such as the opening and closing of valves, sliding sleeves, etc. Furthermore, in many cases, a downhole obstruction may be present in the form of an irreversibly set flapper or isolation valve or other such architectural barrier. While such features may be intentionally locked in place, their removal may nevertheless require a subsequent drill-out or milling intervention.
Drill-out and/or milling removal of isolation valves and other, usually metal-based obstructions, is generally driven by way of a coiled tubing or drill pipe operations. So for example, production operations may be shut down as large scale coiled tubing equipment is delivered at the oilfield and rigged up to the well. A milling tool may then be advanced downhole by way of coiled tubing with a rotatable bit of the tool directed at the isolation valve to achieve its removal. In the case of coiled tubing, 25-50 horsepower or more may be reasonably available for driving such milling. Further, where more power is desired, substantially larger scale drill pipe equipment may be utilized to drive the milling application, such equipment readily supplying horsepower in the hundreds.
Unfortunately, driving of such milling and/or drill-out applications comes at a fairly significant price. Namely, the time required to rig-up and run such large scale applications may be quite costly, not to mention the amount of footspace required to support such equipment. Indeed, in addition to recognizing the significant expenses involved in completions operations as described above, significant efforts have also been directed at cost-reductions for follow-on maintenance applications such as the noted milling and drill-out applications. Thus, recently efforts have been made to allow for delivery and powering of such applications over wireline conveyance.
Wireline delivery of milling and/or drill-out tools involves the rig-up and deployment of much smaller scale wireline equipment, as compared to the above noted coiled tubing or drill pipe deployment equipment. Thus, the time and footspace required for rig-up and running of the application may be dramatically reduced, not to mention the overall manpower required.
Unfortunately, wireline equipment effectively provides a limited amount of horsepower downhole, generally well below 10 horsepower. In circumstances where the equipment is employed to aid in scale removal, such power may be more than adequate. However, as described below, where the application is directed at the removal of isolation valves and other such metal based features, particular challenges may arise that prevent efficient or effective removal with such limited horsepower available.
The rotating bit of a drill-out or milling tool is forcibly driven in a downhole direction by way of an adjacent actuator that includes a reciprocating piston. This piston is itself hydraulically driven. In other words, fluid pumped in and out of a pressurizable housing may be used to reciprocate the piston. However, such fluid is inherently compressible to a certain degree. That is to say, pressure in a chamber of the housing may be driven up to advance the piston. However, such pressure may alternately result in a degree of compression of the fluid itself. To the extent that this occurs, the piston is no longer forcibly driven. Ultimately, this may result in a ‘bounce’ or a certain degree of inconsistency in the driving of the bit relative the obstruction.
Where the obstruction is a metal-based feature, such inconsistent driving or ‘bouncing’ of the milling or drill-out bit may result in cold working and hardening of the feature. This is due to the fact that with less than about 5-10 horsepower available, even a minor degree of bounce is likely to translate into actual intermittent disengagement of the bit relative the feature. As a result, the amount of time required to complete the removal of the feature may be increased dramatically. Such is often the case where the feature is an isolation valve which is often of a metal based superalloy. Furthermore, where a carbide or other sufficiently hard bit is employed, the likelihood of the bit breaking in response to such bouncing and hardening of the valve is quite significant. Indeed, where this occurs, the entire wireline assembly may be removed from the well for bit replacement, thereby adding as much as a day's worth of time to the application. Therefore, at present, wireline deployment of milling and/or drill-out equipment is generally foregone in place of much more expensive and time consuming alternatives.